mlai
Oh this is simple. Webcomics aren't popular just like comics aren't popular. Manga is popular in Korea and by extension web-manga becomes popular enough to enter mainstream media.
The only way a comics can be popular enough in the USA to get a blockbuster movie, is if it's superhero comics, where the plot doesn't matter only the character. And by extension, that means no indie comics becoming popular. The Korean manga "Moss", for example, I'm sure wasn't created and published under the editing heels of some stupid and unhealthy corporation that thought One More Day is a good idea.
It's like asking why Soccer/Football is not very competitive in the USA, despite the country having a spacious land mass and pleasant outdoor weather.
A lot of this is incredibly untrue. Comics and Webcomics are immensely popular. Think about how many people are members of this site and similar sites. Think of all the independently hosted webcomics out there, and all the people that are on the forums, not to mention those who simply read the comic. Comics in general are arguably facing its greatest popularity in 20 years.
As for comics being turned into movies, there are plenty of non-superhero/indie comic book movies. Allow me to begin:
A History of Violence
Ghost World
Persepolis (yeah, it's French but was pretty successful in America)
Scott Pilgrim
Tank Girl (we try to forget about it, but it existed)
American Splendor
And according to Jeff Smith, serious movement towards a Bone movie is happening.
So plenty of Indie comic movies have been made. Sure super-hero movies are more popular, but that's probably just because they make much better summer blockbusters than anything Dan Clowes has put together.
If anything is really holding back webcomics as a respected form, it's simply that comics has never really found it's proper place in America (despite essentially being invented here). In it's history comics have sort of just sat on a fence between the visual and the literary. It wasn't until recent years that comics had a place in the typical bookstore and a gallery showing comic pieces still has that air of "folk" art. Pretty much the entire history of comics in America has been based on some form of controversy.
So when America doesn't know how it really feels about professional print comics, it's not going to embrace the overwhelmingly amateur world of web-comics. That is not to say however that there are not popular as well as strong webcomics, such as Penny Arcade or Girl Genius.
Webcomics themselves are still greatly in their infancy, as they are still widely print style comics in digital form. There is however movement in comics towards creating work that embracing web technology, look up Scott McCloud and his infinite canvas idea it's really impressive.
I'd also like to note that we shouldn't be measuring the value of comics in our country by how often it gets turned into a movie. That'd be a grave mistake. The comic is it's own medium, not some intermediary between painting and motion pictures. There are things you can do with comics that you could never do with movies, and many artists embrace this. I mean, you could not translate Chris Ware's "Jimmy Corrigan: Smartest Kid on Earth" into a movie without losing something incredibly precious to its original creation.